Study of chemical, kinetic, and theoretical sorption properties of activated carbons obtained from agroindustrial origin: comparison of anionic and cationic model molecules

This work conducted an experimental study on the sorption characteristics of seven activated carbons from agroindustrial origin. Activated carbons derived from spent-coffee ground, orange peel, sugarcane bagasse, cedar sawdust, agave bagasse, coconut shell, and citric residue have been prepared with phosphoric acid activation and subsequent carbonization. The obtained materials were characterized using FTIR, SEM, EDS, Boehm potentiometric titration, and PZC measurements. Once the samples were characterized, the sorption of anionic and cationic model molecules was studied using kinetic models and compared to density functional theory calculations. The understanding of the surface characteristics of activated carbons allowed to describe and correlate the adsorption behavior of anionic and cationic model molecules. According to FTIR spectroscopy and Boehm titration results, the surface of the activated carbon substrates has acidic and basic oxygenated functional groups. Adsorption experiments were carried out to compare the sorption of two model molecules: methylene blue and methyl orange as cationic and anionic species, respectively, under near neutral pH, very acidic for methyl orange, and very basic for methylene blue, based on the corresponding pKa. Molecular modeling using DFT was also used to calculate adsorption Gibbs’ free energies for sorption reactions between the model molecules and the oxygenated functional groups. Almost all the free energies were negative which means that adsorption interactions are thermodynamically favorable. As expected for adsorption processes, in all cases, the enthalpic contribution dominated the entropic one. In addition, it is shown that the predominating interactions are H-bonding and π-π interactions. Methylene blue uptakes range from 0.026 mmol/g for graphite to 0.121 mmol/g for coconut shell carbon that corresponds to 24.8% and 100% removal, respectively. Increasing the methylene blue’s pH from 6.6 to 12 raised the cationic molecule uptake by ~ 30%. Furthermore, the coconut shell also removes the higher amount of methyl orange, near 99% at neutral and low pH. Citric residue does not adsorb methyl orange at neutral pH whereas at acidic conditions (pH = 3), it adsorbs 0.011 mmol/g (11% removal); in general, methyl orange adsorption was slightly lower than that of methylene blue.